If you are on sole source then no they are not allowed.
I am on ss + and I have balsamic vinegar, although not in the book my CDC said a little amount of this wouldnt do any harm.
If you dont want to risk it, then no dressing at all
LOL, if you're on Sole Source, you'd have nothing to put a salad dressing on, so I'm guessing you're on SS+, Beanflower.
If you're on SS+ then officially the only things you can add are herbs and spices such as curry or chilli powder, coriander, pepper (as in the stuff you shake on), chives and parsley. No salt.
Dressings aren't on the list until 810 at which point you can have fat/calorie-free non-oil based dressings (e.g. balsamic vinegar) or even a squirt of lemon juice.
Cheers. One of the girls from uni is on ss+ and I forgot to ask my cdc (mine is now on holiday) as hers told her she can have low fat salad dressings. I'll let her know.